Lithium secondary batteries have been used widely as power sources for portable electronic instruments, because they are small-sized and have high energy densities. As for their positive-electrode active materials, lamellar compounds, such as LiCoO2, have been employed mainly. However, these compounds have such a problematic issue that the oxygen is likely to be eliminated before and after 150° C. under the fully-charged conditions so that this is likely to cause the oxidative exothermic reactions of nonaqueous electrolyte liquids.
Recently, as for positive-electrode active material, olivine-type phosphate compounds, Li″M″PO4 (LiMnPO4, LiFePO4, LiCoPO4, and the like), have been proposed. These compounds upgrade the thermal stabilities by means of using the divalent/trivalent oxidation-reduction reaction, instead of the trivalent/tetravalent oxidation-reduction in which an oxide like LiCoO2 serves as a positive-electrode active material; and have been attracting attention as compounds from which higher discharging voltages are available by means of arranging the polyanions of hetero elements whose electronegativities are higher around the central metal.
However, in a positive-electrode material comprising an olivine-type phosphate compounds, its theoretical capacity is limited to 170 mAh/g approximately because of the large molecular weight of phosphate polyanions. Furthermore, LiCoPO4 and LiNiPO4 have such a problem that no electrolytic liquids, which can withstand their charging voltages, are available because the operating voltages are too high.
Hence, as a cathode material that is inexpensive, which is more abundant in the amount of resource, which is lower in the environmental load, which has a higher theoretical charging-discharging capacity of lithium ion, and which does not release any oxygen at the time of high temperature, lithium-silicate-system materials, such as Li2FeSiO4 (with 331.3-mAh/g theoretical capacity) and Li2MnSiO4 (with 333.2-mAh/g theoretical capacity), have been attracting attention. These silicate-system materials are expected as positive-electrode materials for lithium secondary batteries with much higher capacities. In addition, their charging voltages are lower than those of phosphate-system ones by about 0.6 V approximately, which is a reflection of the fact that the electronegativity of Si, a hetero element, is smaller than that of P, and there is such a possibility that Co and Ni are employable as a doping element to the silicates.
Of these silicate materials, Li2FeSiO4 is a material showing the highest charging-discharging characteristic ever that has been reported at present. Although Li2FeSiO4 exhibits a capacity of 160 mAh/g approximately, it has not yet arrived at obtaining a charging-discharging characteristic that exceeds 169.9 mAh/g, the theoretical capacity of LiFePO4 that is one of the current materials.
As for synthesizing methods for the aforementioned silicate-system compounds, the hydrothermal synthesis method, and the solid-phase reaction method have been known. Of these methods, although it is feasible to obtain fine particles with particle diameters of from 1 to 10 nm approximately by means of the hydrothermal synthesis method, there is such a problem that doping elements are less likely to dissolve and so the phases of impurities are likely to be present mixedly.
Meanwhile, in the solid-phase reaction method, although it is feasible to dissolve doping elements because it is needed to cause reactions at such high temperatures as 1,000° C. or more for a long period of time, the resulting crystal grains become larger to 10 μm or more, thereby leading to such a problem that the diffusion of ions is slow. Besides, since the reactions are caused at the high temperatures, the doping elements, which have not dissolved completely, precipitate to generate impurities in the cooling process, and so there is such a problem that the resultant resistance becomes higher. In addition, since lithium-deficient or oxygen-deficient silicate-system compounds have been made due to the heating being done up to the high temperatures, there is also such a problem that it is difficult to increase capacities or to upgrade cyclic characteristics (see following Patent Literature Nos. 1 through 4).